This invention relates to a method for forming a coated film and an apparatus therefor, and more particularly to a method for forming a coated film on a desired region of an inner peripheral surface of a cylinder and an apparatus therefor.
In the prior art, an adhesive applying apparatus typically includes a cylindrical container (cylinder) which receives an adhesive curing upon contact with air, a nozzle arranged at a distal end of the container and a piston member arranged in an opening formed at a rear portion of the container, wherein the piston is driven toward the nozzle to discharge the adhesive from a distal end of the nozzle. In the conventional adhesive applying apparatus thus constructed, in order to prevent air from intruding from an outside of the apparatus through a gap between the piston and the adhesive thereinto immediately before starting of adhesive applying operation of the apparatus, an inner peripheral surface of the container is coated on a portion thereof at which the piston is initially arranged with a seal material so as to extend over a whole circumference thereof. In the prior art, such application of the seal material (thermoplastic material) to at least a part of the inner peripheral surface of the cylinder in a manner to extend over a whole circumference thereof is carried out by inserting a nozzle into an inner space of the cylinder to spray the seal material onto the portion of the inner peripheral surface. Also, such a conventional seal material applying apparatus uses a gear pump for generating a pressure required to feed the seal material in the form of a molten paste to the nozzle.
However, the above-described conventional techniques of forming an applied or coated film of the seal material by spraying cause the molten paste to be scattered to an undesired region of the inner peripheral surface of the cylinder other than a desired one thereof, leading to a failure to form the coated film on the desired region. In order to avoid the problem, techniques of carrying out spraying of the adhesive by means of a mask are proposed. Also, the paste applying apparatus using the gear pump is hard to control a pressure under which the thermoplastic material is discharged from the nozzle, to thereby fail to form the coated film into a uniform thickness.
The present invention has been made in view of the foregoing disadvantage of the prior art.
Accordingly, it is an object of the present invention to provide a coated film forming method which is capable of readily and uniformly coating or depositing a thermoplastic material on a desired region of an inner peripheral surface of a cylinder.
It is another object of the present invention to provide a coated film forming apparatus which is capable of permitting a thermoplastic material to be readily and uniformly coated on or applied to a desired region of a cylinder.
It is a further object of the present invention to provide a coated film forming apparatus which is capable of automatically replenishing a molten paste.
The present invention improves a method for forming a coated film of a thermoplastic material on a desired region of at least a part of an inner peripheral surface of a cylinder to be coated (hereinafter also referred to as xe2x80x9ccoated regionxe2x80x9d) so as to extend in a whole circumferential direction thereof. The term xe2x80x9ccylinderxe2x80x9d as used herein is intended to cover a cylindrical structure having an opening provided on at least one end thereof. The method includes the steps of providing a paste applying machine for discharging a molten paste of the thermoplastic material kept molten by heating from a distal end of a nozzle, arranging the nozzle in a space in the cylinder so that the molten paste is discharged toward the inner peripheral surface of the cylinder, and moving the nozzle along a rotational center of the cylinder and within a range opposite to the region while rotating the cylinder in the circumferential direction and discharging the molten paste from the nozzle. Actually, the nozzle is inserted into the space in the cylinder through the opening thereof. In this instance, rotation of the cylinder in the circumferential direction may be carried out either in a right-hand direction (clockwise direction) or in a left-hand direction (counter-clockwise direction) as viewed from a side of the opening of the cylinder. Also, the nozzle for discharging the molten paste therefrom may be moved either from a position deep in the cylinder toward a side of the opening of the cylinder or from the side of the opening toward the deep position. Movement of the nozzle carried out while rotating it permits the molten paste to be applied in a spiral pattern to the inner peripheral surface of the cylinder. The method further includes the step of spreading the molten paste applied to the inner peripheral surface by means of centrifugal force acting on the cylinder being rotated, to thereby wholly cover the region with the molten paste.
A rotational speed of the cylinder is determined so as to permit adjacent lines of the molten paste spirally applied to be spread by centrifugal force, so that the lines may be joined together and a thickness of a coated film formed may be as uniform as possible. The rotational speed may be kept low during a period of time for which the molten paste is discharged. After the molten paste is discharged onto the coated region, the rotational speed may be increased so as to wholly cover the coated region with the molten paste. Alternatively, of course discharge of the molten paste from the nozzle may be carried out at a high speed from start of the discharge.
The thus-constructed method of the present invention permits the molten paste to be readily and positively applied to any desired coated region of the inner peripheral surface of the cylinder without use of any mask as required in the conventional spraying techniques.
In the present invention, viscosity of the molten paste, a rotational speed of the cylinder and a speed of movement of the nozzle are determined so as to prevent the molten paste discharged onto the inner peripheral surface from the nozzle from being scattered to a region other than the coated region.
The nozzle of the paste applying machine may have a discharge port which can be formed into any suitable shape, provided that it permits the molten paste to be linearly discharged. Typically, the discharge port of the nozzle may be formed into a substantially circle shape. In this instance, when the molten paste has viscosity set to be within a range of between 50 cp and 100 cp, the molten paste may be discharged from the nozzle under a pressure of 1 kg/cm2 or less under the conditions that a rotational speed of the cylinder is set to be within a range of between 2700 rpm and 3300 rpm, a speed of movement of the nozzle is set to be within a range of between 0.055 m/s and 0.08 m/s and a distance between the distal end of the nozzle and the inner peripheral surface of the cylinder is set to be within a range of between 3 mm and 7 mm. Such configuration positively keeps the molten paste discharged from the nozzle from being scattered to a region other than the coated region.
In addition, the present invention provides an apparatus for forming a coated film of a thermoplastic material on a region of at least a part of an inner peripheral surface of a cylinder so as to extend in a whole circumferential direction thereof. The apparatus includes a cylinder drive mechanism for rotating the cylinder in the circumferential direction about a central line of the cylinder, a paste applying machine for discharging a molten paste of the thermoplastic material kept molten by heating from a distal end of a nozzle, and a timing controller. The paste applying machine includes a gun head provided with the nozzle, a gun head moving mechanism for moving the gun head and a molten paste feed equipment for feeding the molten paste to the gun head. The timing controller is constructed in such a manner that operation timing of each of the cylinder drive mechanism, gun head moving mechanism, and molten paste feed equipment is determined so as to permit the cylinder to be rotated in the circumferential direction while keeping the nozzle arranged in a space in the cylinder and so as to permit the nozzle to be moved along a rotational center of the cylinder being rotated and within a range opposite to the region while keeping the molten paste discharged from the nozzle. Such construction permits a speed of movement of the gun head, a rotational speed of the cylinder and discharge of the molten paste to be readily optimized.
The molten paste feed equipment includes a molten paste feed unit (a feed change-over module and a pressure pump unit) which includes a storage tank in which the molten paste is stored and feeds the molten paste to the gun head under a predetermined pressure so as to permit the molten paste to be discharged from the nozzle under the predetermined pressure, and a molten paste replenishing unit for automatically replenishing the molten paste to the storage tank of the molten paste feed unit when the amount of molten paste in the storage tank of the molten paste feed unit is reduced to a level lower than a predetermined level.
The molten paste feed unit is constructed so as to keep a pressure in the storage tank at a constant level, so that the pressure in the storage tank permits the molten paste to be fed to the gun head. The molten paste in the molten paste replenishing unit is fed to the storage tank under a pressure which is set to be higher than the pressure in the storage tank. The storage tank of the molten paste feed unit is provided therein with a level sensor for detecting a level of the molten paste therein. The storage tank has a molten paste replenishing port provided with a control on/off valve which is kept open during a period of time for which a control command is inputted thereto and kept closed during the remaining period of time. The control on/off valve of the molten paste feed unit outputs the control command during a period of time defined between after the level sensor detects that a level of the molten paste in the storage tank of the molten paste feed unit is at a first level or below and before it detects that the level of the molten paste in the storage tank reaches a second level higher than the first level.
Such construction ensures that the molten paste is automatically replenished from the molten paste replenishing unit to the storage tank when the amount of molten paste stored in the storage tank is reduced due to an increase in discharge of the molten paste from the nozzle.
The molten paste feed unit is provided with an on/off valve, which is opened or closed by a command from the timing controller, in the midst of a molten paste feed pipe which connects the storage tank and the gun head. Controlling this on/off valve permits feed of the molten paste to the gun head to be positively managed.